Thermal spraying processes are generally used in order to generate cost-effective layers of components to be coated or to provide these with properties which cannot otherwise be generated. For this purpose, the layer material has to be fed into the spraying process, this usually taking the form of particles. These particles are conducted through a supply line which they leave through a mouth in order to be picked up by a carrier gas stream which, for coating purposes, is directed onto the component to be coated. So that the particles adhere to the component to be coated, these must have imparted to them an energy amount which is dependent on the coating method and material and which causes the particles to adhere to the component to be coated. This introduction of energy may take place, for example, by heating the particles during spraying or else by accelerating the particles. In cold-gas spraying, however, the kinetic energy introduced into the process as a result of acceleration is converted into deformation or heat when the particles impinge on the component to be coated. If there is a sufficient introduction of energy, heating of the particles leads to a softening or even a melting of the particles, thus facilitating an adhesion of the particles impinging onto the component to be coated.
In cold-gas spraying, an introduction of energy in the form of kinetic energy is adopted primarily, although an additional heating of the particles may take place, but this does not usually cause a fusion or melting of the particles. On account of the high kinetic energy of the particles, these experience plastic deformation when they impinge onto the surface to be coated, a simultaneous deformation of the surface causing an adhesion of the particles. Furthermore, for example, high-velocity flame spraying makes available a thermal spraying method in which both the kinetic energy and the thermal energy of the particles impinging onto the surface to be coated play an appreciable part in layer formation. Cold-gas spraying is mentioned, for example in DE 197 47 386 A1.
To achieve a high-quality coating result, it is particularly important that the particles provided for coating can be delivered to the carrier gas stream in a clearly defined way. In order to ensure this, in particular, an agglomeration of the particles must be suppressed, so that these can be fed into the carrier gas stream as uniformly as possible and not as large clusters. As may be gathered from U.S. Pat. No. 6,715,640 B2, an agglomeration of the coating particles can be reduced or canceled, for example, by mechanical means. The particles are in this case stored in a funnel-shaped container and are extracted from this in the quantity required in each case. The extracted quantity can be treated by vibration and agitation in such a way that a separation of the particles takes place and these can be delivered to a transport gas. This gives rise to a particle/gas mixture which can be delivered to the carrier gas stream of a thermal spraying process through a supply line.
A. Killinger et al, “High-Velocity Suspension Flame Spraying (HVSFS), a new approach for spraying nanoparticles with hypersonic speed”, Surface & Coatings Technology 201 (2006) 1922-1929, and U.S. Pat. No. 6,579,573 B2, U.S. Pat. No. 6,491,967 B1, EP 1 134 302 A1 and DE 103 92 691 T5 disclose thermal coating methods in which the introduction of energy into the jet containing the coating particles takes place by means of a flame, such as, for example, a plasma flame. In this flame spraying coating method, the adhesion of the coating particles on the substrate to be coated is ensured by means of the flame as an energy source with a relatively high energy density. This energy source is in the form of a flame in the center of a coating nozzle, so that coating particles in the form of a liquid dispersion can be delivered directly to the flame. The high energy density of the flame in this case ensures a complete evaporation of the dispersant, while the energy amount necessary for evaporation can be made available by suitably regulating the energy supply for the flame. The flame, because of the high energy density, can readily make available the energy amount necessary for the evaporation of the dispersant.